1. Field of the Invention
The present invention relates to a switching power source device for supplying a DC stabilized voltage to industrial or household electronic appliances.
2. Description of the Related Art
In recent years, with development of low price, downsizing, high performance and energy saving in electronic equipment, a switching power source device has been required to be small in size, high in output stability and high in efficiency. Prior art switching power source devices will be explained below.
Traditionally, for such a switching power source device, a self-exciting flyback type switching power source device, which has few components and can be fabricated at low cost, has been generally used widely. However, as well known, this type of device has the problem that the switching frequency is greatly changed by an output current so that interference for electronic equipment is generated and a large rectifying and smoothing circuit is required.
In order to solve the above problem in the prior art, a regenerative controlled type of switching power source device as shown in FIG. 9 has been developed. In FIG. 9, reference numeral 1 denotes an input DC power source which can be obtained by rectifying and smoothing an AC voltage or may be constructed by a battery. The DC power source 1 supplies an input voltage between input terminals 2 and 2' so that a positive voltage is connected with the input terminal 2 while a negative voltage is connected with the input terminal 2'. Reference numeral 3 denotes a transformer in which a primary winding 3a has one end connected with the input terminal 2 and the other end connected with the input terminal 2' through a switching element 4, a secondary winding 3c has one end connected with an output terminal 10' and the other end connected with an output terminal 10 through a diode 7, and a bias winding 3b has one end connected with the input terminal 2' and the other end connected with a sync oscillator circuit 6. Reference numeral 4 denotes the switching element which is turned on or off by the on/off signal supplied to its control terminal from the sync oscillator circuit 6 to apply the input voltage to the primary winding 3a or cut off it.
The sync oscillator 6 causes the switching element 4 to perform an ON operation during its predetermined ON period and causes the element to perform an OFF operation so that its OFF period continues until the polarity of the voltage induced in the bias winding 3b is reversed, and continues oscillation by repetition of the ON/OFF operation. Reference numeral 18 denotes a secondary switching element. The energy stored in the transformer 3 during the ON period of the switching element 4 is discharged, during the OFF period of the switching element 4, through the secondary winding 3c from a rectifying diode 7 or the secondary switching element 18 into a smoothing capacitor 8. Thereafter, in reverse, a secondary current is caused to flow backward from the smoothing capacitor 8 to the secondary winding 3c through the secondary switching element 18. With the aid of a control circuit 19, the secondary switching element 18 serves to control the backflow period while the secondary current flows. Reference numeral 7 denotes a rectifying diode with its anode side connected with the one end of the secondary winding 3c and its cathode side connected with an output terminal 10. Reference numeral 8 denotes a smoothing capacitor which is connected between the output terminals 10 and 10'. The rectifying diode 7 rectifies the voltage induced in the secondary winding 3c and the smoothing capacitor 8 smooths the rectified voltage to provide an output voltage. The control circuit 19 detects the output voltage between the output terminals 10 and 10' and compares it with an internal reference voltage to change the backflow period while the secondary current is caused to flow through the secondary switching element 18.
Now also referring to FIG. 10, a detailed explanation will be given of the operation of the device of FIG. 9. In FIG. 10, (a) shows the voltage waveform V.sub.DS across the switching element 4; (b) shows the waveform of the primary current flowing through the primary winding 3a; (c) shows the waveform of the driving pulse V.sub.G1 in the sync oscillator circuit 6; (d) shows the waveform of the secondary current I.sub.D flowing through the second winding 3c; (e) shows the waveform of the driving pulse V.sub.G2 for the secondary switching element 18 in which the shaded period during the OFF period represents the backflow period while the secondary current is caused to flow toward the secondary winding 3c.
The primary current flowing through the primary winding 3a during the ON period of the switching element 4, which operates during the ON period determined by the sync oscillator circuit 6, generates magnetic flux in the transformer 3 to store energy in it. Then, an induced voltage is generated in the secondary winding 3c of the transformer 3. The induced voltage is so adapted that it reverse-biases the rectifying diode 7 and places the secondary switching element 18 in an n"OFF" state.
When the OFF signal from the sync oscillator circuit 6 turns off the switching element 4, a flyback voltage is generated in the primary winding 3a and also generated in the secondary winding 3c. As a result, the voltage is applied to the rectifying diode 7 in its forward-biasing direction so that the energy stored in the transformer 3 is discharged as a secondary current through the secondary winding 3c and smoothed by the smoothing capacitor 8 so that the resultant voltage is applied between the output terminals 10 and 10' as an output voltage. Then, the control circuit 19 turns on the second switching element 18, but no particular change in the operation occurs according to the path through which the secondary current flows.
When the energy stored in the transformer 3 is discharged completely so that the secondary current becomes zero, the voltage across the smoothing capacitor 8, i.e., the output voltage is applied to the secondary winding 3c through the secondary switching element 18 already turned on. Thus, the secondary current flows in a backward direction from the smoothing capacitor 8 so that the magnetic flux in the direction opposite to the previous case is generated in the transformer 3 so as to store energy in it.
In this state, the polarity of the induced voltage in each of the wirings of the transformer 3 does not change so that the flyback voltage in the bias winding 3b does not also change. The sync oscillator circuit 6 maintains the OFF period of the switching element 4. When the secondary switching element 18, the ON period of which is controlled by the control circuit 19, turns off, the polarity of the induced voltage generated in each of the winding is reversed. Thus, the induced voltage generated in the secondary winding 3c reverse-biases the rectifying diode 7. Then, the secondary switching element 18 is also "OFF". So the secondary winding current ceases to flow. The induced voltage in the primary winding 3a is generated in such a direction that the connection end of the switching element 4 is at a negative voltage and that of the input terminal 2 is at a positive voltage. Thus, the primary current flows in such a direction that the input DC power source 1 is charged through the diode 5 so that the energy in the transformer 3 stored during the OFF period is returned as power to the input DC power source 1.
Then, since the polarity of the induced voltage generated in the bias winding 3b changes, the sync oscillator circuit 6 turns on the switching element 4, but no particular change in the operation occurs according to the path through which the primary current flows. When the energy stored in the transformer 3 during the OFF period is discharged completely so that the primary current becomes zero, the primary current flows in the charging direction opposite to the above case from the input DC power source 1 through the switching element 4 already turned on. Thus, magnetic flux is generated in the transformer 3 so as to store energy in it. In this state, the polarity of the induced voltage generated in each of the windings of the transformer 3 is not changed so that the sync oscillator circuit 6 maintains the 0N state of the switching element 4. When the switching element 4 which operates during the ON period determined by the sync oscillator 6 turns off, the energy stored in the transformer 3 is discharged as the secondary current through the secondary winding 3c. If the above operations are repeated, the output voltage is continuously supplied from the output terminals 10 and 10'.
A detailed explanation will be given of the operation of controlling the output voltage so as to be stable. FIG. 10 shows respective operation waveforms of the indicated voltages and currents. Now it is assumed that the. OFF period (t1-t3) of the driving pulse waveform V.sub.G1 in the sync oscillator circuit 6 is T.sub.OFF and the backflow period of the secondary current I.sub.O within it is T'.sub.OFF while the ON period thereof (t3-t5) is T.sub.ON and the return period (t3-t4) of the primary current I.sub.D within it is T'.sub.ON . The output current I.sub.OUT supplied from the output terminals 10-10' can be described by ##EQU1## The output voltage V.sub.OUT can be represented by ##EQU2## The oscillation frequency f can be described by ##EQU3## In these equations, N.sub.S denotes the number of wirings of the secondary winding 3c, N.sub.P denotes the number of windings of the primary winding 3a, L.sub.S denotes the inductance value of the secondary winding 3c, V.sub.IN denotes the input voltage supplied from the DC power source 1, T.sub.ON denotes the ON period of the switching element 4, T.sub.OFF denotes the OFF period of the switching element 4 and T denotes an oscillation period.
Since the ON period T.sub.ON is fixed to the constant value determined by the sync oscillator circuit 6, if the output voltage V.sub.OUT is constant, the OFF period is constant and the oscillation frequency is also constant. However, since the backflow period T'.sub.OFF can be varied by the switching element 18 which is controlled by the control circuit 19, when the output current I.sub.OUT varies, the output voltage V.sub.OUT can be controlled by varying the backflow period T'.sub.OFF as understood from Equation (1). The output voltage V.sub.OUT can be also controlled for a variation in the input voltage V.sub.IN by varying the backflow period T'.sub.OFF as understood from Equation (2). Thus, the control voltage V.sub.OUT can be controlled so as to be always constant by controlling the backflow period T'.sub.OFF in such a manner that the ON period of the secondary switching element 18 is controlled by the control circuit 19.
FIG. 11 corresponding to FIG. 10 show respective operation waveforms when the output current I.sub.OUT varies. In FIG. 11, like parts which will not be explained here refer to like parts in FIG. 10. In FIG. 11, solid lines refer to a so-called maximum load case when the maximum output current I.sub.OUT flows from the output terminals 10-10' whereas broken lines refer to a so-called no load case when the output current I.sub.OUT is zero. If the input voltage is constant, the T.sub.ON period is also constant so that the changing width .DELTA.B in the magnetic flux is constant.
When the switching element 4 turns off, the surge voltage due to the leakage inductance in the transformer 3 is generated. Where the maximum load is applied, the degree of the surge voltage is similar to the case of the conventional flyback-type switching power source device; on the other hand, where small load is applied, the peak value of the primary current immediately before turn-off is large so that it is larger than the above conventional power source device.
In the case of the regenerative type switching power source device, because of the energy regenerative capability of the switching element 4 when it turns on, the snubber capacitor connected across the switching element 4 does not constitute a turn-on loss but can effectively restrain the surge voltage in the turn-on. But, resonance energy of the snubber capacitor and the leakage inductance in the transformer 3 becomes large so that a ringing waveform is superposed on the voltage across the switching element 4 during the OFF period and constitutes a noise source. The addition of such a snubber capacitor having a larger capacitance hinders the switching frequency from being higher frequency desirable for downsizing the power source.
As the second prior art, the switching power supply device in a primary side regenerative system as shown in FIG. 12 has been developed. In FIG. 12, like reference numerals which will not explained here in detail designate like parts in FIG. 9 described above. Numeral 1 denotes a DC power source; 2-2' input terminals; 3 a transformer having a primary winding 3a, a secondary winding 3b and a bias winding 3c; 4 a switching element; 5 a diode; 6 a sync oscillator circuit; 7 a rectifying diode; and 8 a smoothing capacitor. The diode 7 and smoothing capacitor 8 constitute a first rectifying and smoothing circuit. Numeral 9 denotes a control circuit and 10-10' denote output terminals.
Numeral 12 denotes a rectifying diode and 13 denotes a smoothing capacitor. The rectifying diode 12 has an anode connected with the connection point between the primary winding 3a and the switching element 4 and a cathode connected with the one end of the smoothing capacitor 13 so that the primary winding 3a, rectifying diode 12 and smoothing capacitor 13 constitute a closed circuit. Numeral 11 denotes a second switching element which is connected in parallel with the rectifying diode 12 and on-off controlled by the control circuit 9. Incidentally, within the control circuit 9, the part connected with the output terminals and the part for driving the switching element 11 are separated from each other.
Referring to FIG. 13 showing the waveform charts at the various parts of the switching power source device, its operation will be explained. In FIG. 13, (a) shows the voltage waveform V.sub.DS across the switching element 4; (b) shows the waveform of the primary current flowing I.sub.D through the switching element 4 or diode 5; (c) shows the waveform of the driving pulse V.sub.G1 in the sync oscillator circuit 6; (d) shows the waveform of the primary current I.sub.C flowing through the switching element 11 or rectifying diode 12; (e) shows the waveform of the driving pulse V.sub.G2 for the switching element 11; (f) shows the secondary current I.sub.O flowing through the secondary winding 3c; and (g) shows the changes in the magnetic flux .PHI. in the transformer 3.
The primary current I.sub.D flowing through the primary winding 3a during the ON period of the switching element 4, which operates during the ON period determined by the sync oscillator circuit 6, generates magnetic flux in the transformer 3 to store energy in it. Then, an induced voltage is generated in the secondary winding 3c of the transformer 3. The induced voltage is so adapted that it reverse-biases the rectifying diode 7. The rectifying diode 12 is also reverse-biased and the switching element 11 in an "OFF" state.
When the OFF signal from the sync oscillator circuit 6 turns off the switching element 4, a flyback voltage is generated in the primary winding 3a to forward-bias the rectifying diode 12 and also generated in the secondary winding 3c so that the voltage is applied to the rectifying diode 7 in its forward-biasing direction. Thus, the energy stored in the transformer 3 is discharged as a primary current I.sub.C through the primary winding 3a and rectifying diode 12 and the current is smoothed by the smoothing capacitor 13 to provide a DC voltage V.sub.C. The energy is also discharged as a secondary current I.sub.O through the secondary winding 3c and the current is smoothed by the smoothing capacitor 8 to be supplied to the output terminals 10-10' as an output voltage V.sub.OUT.
Then, the control circuit 9 turns on the second switching element 11, but any particular change in the operation does not occur if the primary current I.sub.D flows through either the rectifying diode 12 or switching element 11. Taking no account of the capacitance component such as parasitic capacitance, when the switching element 4 turns off and the polarity of the voltage in each of the windings of the transformer 3 is reversed, the energy stored in the transformer 3 is discharged first from the primary winding 3a under the influence of leakage inductance. Specifically, the primary current I.sub.C start to flow from the initial value of the final value I.sub.P of the primary current I.sub.D, and the secondary current I.sub.O rises from zero.
Then, the magnetic flux .PHI. in the transformer 3 decrease linearly because the stored energy is discharged with the DC voltage applied to the primary winding 3a. Correspondingly, the primary current I.sub.C decreases monotonously and eventually becomes 0 A. But since the switching element 11 is "ON", now conversely, the current discharged from the smoothing capacitor 13 flows through the switching element 11. Since the DC voltage V.sub.C has been applied to the primary winding 3a, the rectifying diode 7 is forward-biased and so the secondary current I.sub.O continues to flow. Also after the energy stored in the transformer 3 has been discharged during the ON period of the switching element 4, the DC voltage V.sub.C is applied through the switching element 11 to the transformer 3 so as to be reversely excited to store energy in the reverse direction.
When the switching element 11 is turned off the control circuit 9, the polarity of the voltage in each of the winding is reversed. Thus, the rectifying diode 7 is reverse-biased and so the secondary current I.sub.O ceases to flow. The induced voltage in the primary winding 3a is generated in such a direction that the connection end of the switching element 4 is at a negative voltage and that of the input terminal 2 is at a positive voltage. Thus, the primary current I.sub.D flows in such a direction that the input DC power source 1 is charged through the diode 5 so that the energy in the transformer 3 stored during the OFF period is returned as power to the input DC power source 1.
Then, since the polarity of the induced voltage generated in the bias winding 3b changes, the sync oscillator circuit 6 turns on the switching element 4, but no particular change in the operation occurs according to the path through which the primary current I.sub.D flows. When the energy stored in the transformer 3 during the OFF period is discharged completely so that the primary current becomes zero, the primary current I.sub.D flows in the charging direction opposite to the above case from the input DC power source 1 through the switching element 4 already turned on. Thus, magnetic flux is generated in the transformer 3 so as to store energy in it.
In this state, the polarity of the induced voltage generated in each of the windings of the transformer 3 is not changed so that the sync oscillator circuit 6 maintains the ON state of the switching element 4. When the switching element 4 which operates during the ON period determined by the sync oscillator circuit 6 turns off, the energy stored in the transformer 3 is discharged toward the smoothing capacitor 13 through the primary winding 3a and as the secondary current I.sub.0 through the secondary winding 3c. If the above operations are repeated, the output voltage is continuously supplied from the output terminals 10-10'.
A detailed explanation will be given of the operation of controlling the output voltage so as to be stable. FIG. 13 shows various operation waveforms. Now it is assumed that the OFF period (t1-t3) of the driving pulse waveform V.sub.G1 in the sync oscillator circuit 6 is T.sub.OFF and the reverse excitation period (t2-t3) of the transformer 3 within T.sub.OFF is T'.sub.OFF while the ON period thereof (t3-t5) is T.sub.ON and the regenerative period (t3-t4) of the primary current I.sub.D within T.sub.ON is T'.sub.ON. During the stabilizing operation of the switching power source device according to the present invention, the DC voltage V.sub.C does not almost vary because the capacitance of the smoothing capacitor 13 is sufficiently large, and the ripple current therefrom, i.e., the primary current I.sub.ON during the OFF period is 0A in average because the charging and discharging current are equal to each other. Thus, the energy from the second wiring 3c and supplied from the output terminals 10-10' is equal to a difference between the energy stored in the transformer 3 during the ON period and the energy returned to the input DC power source 1 during the T'.sub.ON period.
On the other hand, it is apparent that the DC voltage V.sub.C is described by the following Equation (4) as long as the DC voltage V.sub.C is taken as the output voltage with no load in the operation of stabilizing the output voltage from the regenerative control type switching power source device which has been explained as the prior art. ##EQU4##
Further, since the output voltage V.sub.OUT from the switching power source, which is obtained by rectifying the flyback voltage in the secondary winding 3c, can be regulated by changing the DC voltage V.sub.C as described by Equation (5) ##EQU5##
For example, when the output current I.sub.OUT decreases and the output voltage V.sub.OUT increases, the ON period of the switching element 11 (i.e., the OFF period T.sub.OFF of the switching element 4) is lengthened by the control circuit 9, thus the DC voltage V.sub.C decreases because the charge discharged from the smoothing capacitor 13 is larger than the charge charged therein. While the output voltage V.sub.OUT decreases corresponding to decrease of the DC voltage V.sub.C, the gradient of the primary current I.sub.C is relaxed because the voltage V.sub.C generated in and applied to the winding of the transformer 3 during the OFF period decreases. Thus, the DC voltage V.sub.C eventually settles down at such a value that the output voltage V.sub.OUT becomes a prescribed voltage. As a result, the output voltage V.sub.OUT can be stabilized by regulating the ON period of the switching element 11.
The DC voltage V.sub.C requires essentially small change for correcting the change (load regulation) in the output voltage V.sub.OUT due to the change in the output current I.sub.OUT. Thus, if the ON period T.sub.ON is fixed, the OFF period T.sub.OFF does not almost vary, and the switching frequency and the changing width .DELTA.B in the magnetic flux are also almost fixed. This way is shown in broken lines in FIG. 13. The above arrangement of this prior art can efficiently restrain the surge voltage in turn-off of the switching element and ringing waveform during the OFF period without impairing the efficiency of the regenerative control type switching power source device which can restrain a change in the switching frequency due to the change in load.
But, the prior art arrangements have the following defects. The current flowing through the secondary winding 3c of the transformer 3 and the diode 7 rises from zero in turn-off of the switching element 4 to supply power to the smoothing capacitor 8 and output terminals 10-10'. This current becomes instantaneously zero when the diode 7 is turned off by the voltage induced in the secondary winding 3c of the transformer 3 simultaneously with turn-off of the switching element 11. Then, recovery occurs in the diode 7; it constitutes a noise source and also provides loss which hinders a desirable higher switching frequency required for downsizing the power source.